A
volcano is a mountain that opens downward to a pool of molten rock below the
surface of the earth. When pressure builds up, eruptions occur. Gases and rock
shoot up through the opening and spill over or fill the air with lava fragments.
Eruptions can cause lateral blasts, lava flows, hot ash flows, mudslides,
avalanches, falling ash and floods. Volcano eruptions have been known to knock
down entire forests. An erupting volcano can trigger tsunamis, flashfloods,
earthquakes, mudflows and rockfalls. More than 80 percent of the earth's surface
is volcanic in origin. The sea floor and some mountains were formed by countless
volcanic eruptions. Gaseous emissions from volcano formed the earth's
atmosphere.
The word volcano comes from the little island of Vulcano in the
Mediterranean Sea off Sicily. Centuries ago, the people living in this area
believed that Vulcano was the chimney of the forge of Vulcan( Lat. Volcanus)--the
blacksmith of the Roman gods.
They
thought that the hot lava fragments and clouds of dust erupting from Vulcano
came from Vulcan's forge as he beat out thunderbolts for Jupiter, king of the
gods, and weapons for Mars, the god of war.
Painting
by Herb Kawainui Kane
In
Hawaii the people attributed eruptive activity to the beautiful but wrathful
Pele, Goddess of Volcanoes, whenever she was angry or spiteful.
This
cross-section shows the parts of a volcano. A conduit feeds magma to the
surface. Near the surface, the gas expands and fragments the lava into ash. Some
magma passes through dikes to feed vents. Some magma intrudes parallel to layers
to make sills.
More
than 500 volcanoes are known to have erupted on the earth’s surface since
historic times, and many more have erupted on the ocean floor unobserved by
humans.
Fifty volcanoes have erupted in the United States, which ranks third,
behind Indonesia and Japan, in the number of historically active volcanoes. Of
the world’s active volcanoes, more than half are found around the perimeter of
the Pacific, about a third on midoceanic islands and in an arc along the south
of the Indonesian islands, and about a tenth in the Mediterranean area, Africa,
and Asia Minor.
Volcanoes
Around The World
Smithsonian
Institution, Global Volcanism Program
The
top part of Earth's mantle (called the asthenosphere) contains concentrations of
heavy, radioactive elements such as uranium and thorium.
USGS
Graphic
These isotopes decay
over geologic time into lighter, more stable elements. With this transformation,
a large amount of energy is released to the surroundings, raising the
temperature. For billions of years, this heat energy has been gradually moving
from this zone to the cooler crust.
USGS
Graphic
The
general transportation mechanism of this heat includes super-heated water, gas
venting, and liquid rock and ash extrusion through the surface, termed
volcanism. Places on the Earth where this occurs are called volcanoes, and their
characteristics vary, depending on size, type, location, and activity. Active
volcanoes today exist in the cold Antarctic, under the surface of the Pacific
Ocean, on tropical islands, and in desolate deserts. Some emit steam and
searing-hot noxious gases, some eject choking and blinding, sunlight-blocking
ash; others liquid rock called lava. Many exhibit a combination of these.
Types
Of Volcanoes
Geologists generally group volcanoes into four main kinds--cinder cones,
composite volcanoes, shield volcanoes, and lava domes.
USGS
Graphic
Cinder
Cones
Cinder
cones are the simplest type of volcano. They are built from particles and blobs
of congealed lava ejected from a single vent. As the gas-charged lava is blown
violently into the air, it breaks into small fragments that solidify and fall as
cinders around the vent to form a circular or oval cone. Most cinder cones have
a bowl-shaped crater at the summit and rarely rise more than a thousand feet or
so above their surroundings. Cinder cones are numerous in western North America
as well as throughout other volcanic terrains of the world.
USGS
Graphic
Composite
Volcanoes
Composite volcanoes Some of the Earth's grandest mountains are composite
volcanoes--sometimes called stratovolcanoes. They are typically steep-sided,
symmetrical cones of large dimension built of alternating layers of lava flows,
volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet
above their bases. Some of the most conspicuous and beautiful mountains in the
world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in
Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens
and Mount Rainier in Washington.
Most composite volcanoes have a crater at the summit which contains a central
vent or a clustered group of vents. Lavas either flow through breaks in the
crater wall or issue from fissures on the flanks of the cone. Lava, solidified
within the fissures, forms dikes that act as ribs which greatly strengthen the
cone.
The essential feature of a composite volcano is a conduit system through which
magma from a reservoir deep in the Earth's crust rises to the surface. The
volcano is built up by the accumulation of material erupted through the conduit
and increases in size as lava, cinders, ash, etc., are added to its slopes. When
a composite volcano becomes dormant, erosion begins to destroy the cone. As the
cone is stripped away, the hardened magma filling the conduit (the volcanic
plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by
erosion. Finally, all that remains is the plug and dike complex projecting above
the land surface--a telltale remnant of the vanished volcano.
Shield
Volcanoes
Shield volcanoes, the third type of volcano, are built almost entirely of
fluid lava flows. Flow after flow pours out in all directions from a central
summit vent, or group of vents, building a broad, gently sloping cone of flat,
domical shape, with a profile much like that of a warrior's shield. They are
built up slowly by the accretion of thousands of highly fluid lava flows called
basalt lava that spread widely over great distances, and then cool as thin,
gently dipping sheets. Lavas also commonly erupt from vents along fractures
(rift zones) that develop on the flanks of the cone. Some of the largest
volcanoes in the world are shield volcanoes. In northern California and Oregon,
many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to
2,000 feet. The Hawaiian Islands are composed of linear chains of these
volcanoes including Kilauea and Mauna Loa on the island of Hawaii-- two of the
world's most active volcanoes. The floor of the ocean is more than 15,000 feet
deep at the bases of the islands. As Mauna Loa, the largest of the shield
volcanoes (and also the world's largest active volcano), projects 13,677 feet
above sea level, its top is over 28,000 feet above the deep ocean floor. In some
eruptions, basaltic lava pours out quietly from long fissures instead of central
vents and floods the surrounding countryside with lava flow upon lava flow,
forming broad plateaus. Lava plateaus of this type can be seen in Iceland,
southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake
River in Idaho, and the Columbia River in Washington and Oregon, these lava
flows are beautifully exposed and measure more than a mile in total thickness.
Lava
Domes
Volcanic or lava domes are formed by relatively small, bulbous masses of
lava too viscous to flow any great distance; consequently, on extrusion, the
lava piles over and around its vent. A dome grows largely by expansion from
within. As it grows its outer surface cools and hardens, then shatters, spilling
loose fragments down its sides. Some domes form craggy knobs or spines over the
volcanic vent, whereas others form short, steep-sided lava flows known as
"coulees." Volcanic domes commonly occur within the craters or on the
flanks of large composite volcanoes. The nearly circular Novarupta Dome that
formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet
across and 200 feet high. The internal structure of this dome--defined by
layering of lava fanning upward and outward from the center--indicates that it
grew largely by expansion from within. Mont Pelée in Martinique, Lesser
Antilles, and Lassen Peak and Mono domes in California are examples of lava
domes.
Deep
inside Earth, between the molten iron core and the thin crust at the surface,
there is a solid body of rock called the mantle. When rock from the mantle
melts, moves to the surface through the crust, and releases pent-up gases,
volcanoes erupt. Extremely high temperature and pressure cause the rock to melt
and become liquid rock or magma. When a large body of magma has formed, it rises
through the denser rock layers toward Earth's surface. Magma that has reached
the surface is called lava. Magma comes in different compositions. Each of these
will produce a different lava, from fluid, fast-moving basalt to slower, more
viscous andesite. Because rocks are made up of collections of minerals that melt
at different temperatures, the makeup of the rock being melted affects the magma
that results.
Volcanoes
and Climate Change
Volcanic
eruptions are thought to be responsible for the global cooling that has been
observed for a few years after a major eruption. The amount and global extent of
the cooling depend on the force of the eruption and, possibly, its latitude.
When large masses of gases from the eruption reach the stratosphere, they can
produce a large, widespread cooling effect.
A
volcanic winter is the reduction in temperature caused by volcanic ash and
droplets of sulfuric acid obscuring the sun, usually following a volcanic
eruption.
The
inner workings of "volcanic winter," from Robock, Alan Copyright 2000
AGU
A paper written by Benjamin
Franklin in 1783 blamed the unusually cool summer of 1783 on volcanic dust
coming from Iceland, where the eruption of Laki volcano had released enormous
amounts of sulfur dioxide, resulting in the death of much of the island's
livestock and a catastrophic famine which killed a quarter of the population.
Temperatures in the northern hemisphere dropped by about 1 °C in the year
following the Laki eruption.
Known as the “year without a
summer,” the summer of 1816,
unexpected climate changes left countries in the Northern Hemisphere suffering
from devastating famine and epidemic outbreaks. These weather patterns were the
result of the volcanic eruption of Mount Tambora in Sumbawa, Indonesia, on 10th
April 1815.
It snowed in June in the United
States and Europe. Crops failed, there was starvation, people lost their farms,
and it touched off the wave of emigration that led to the settlement of what is
now the American Midwest. In the meantime, hundreds of thousands more starved
around the world.
New England and Europe were hit
exceptionally hard. Snowfalls and frost occurred in June, July and August and
all but the hardiest grains were destroyed. Destruction of the corn crop forced
farmers to slaughter their animals. Soup kitchens were opened to feed the
hungry. Sea ice migrated across Atlantic shipping lanes, and alpine glaciers
advanced down mountain slopes to exceptionally low elevations.
In 1991 explosion of Mount
Pinatubo, another stratovolcano in the Philippines, cooled global temperatures
for about 2–3 years, interrupting the trend of global warming which had been
evident since about 1970.
The
more than 500 active volcanoes erupting around the world affect the Earth's
atmosphere and global climate. As volcanoes erupt, they blast large clouds of
gases, particles, water vapor, and aerosols into the atmosphere.
Another
possible effect of a volcanic eruption is the destruction of stratospheric
ozone. Researchers now are suggesting that ice particles containing sulfuric
acid from volcanic emissions may contribute to ozone loss. When chlorine
compounds resulting from the breakup of chlorofluorocarbons (CFCs) in the
stratosphere are present, the sulfate particles may serve to convert them into
more active forms that may cause more rapid ozone depletion.
Volcanic
Ozone Hole
"A
'volcanic ozone hole' is likely to occur over the Arctic within the next 30
years," said Azadeh Tabazadeh, lead author of the paper and a scientist at
NASA's Ames Research Center, Moffett Field, Calif. "If a period of high
volcanic activity coincides with a series of cold Arctic winters, then a
springtime Arctic ozone hole may reappear for a number of consecutive years,
resembling the pattern seen in the Antarctic every spring since the 1980s,"
Tabazadeh said. Full story click here
Data
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and
other sources as stated and credited Researched by Charles
Welch-Updated dailyThis
Website is a project of the The Ozone Hole Inc. a 501(c)(3) Nonprofit
Organization
